The present invention relates to the formation of a coating having a cermet structure and devices useable therewith, preferred uses being for optical data recording.
It is known that data can be recorded by selectively illuminating a material so as to selectively change a reflectance of specific portions of the material, such illumination being done using high powered lasers. To read the data, the material is later illuminated with a low powered laser, and differences in reflection from various bits of the material are detected as either a "1" or a "0".
Materials used for this purpose suffer several drawbacks since oftentimes relatively high power densities are required to adequately change a reflectance of a portion of the material so as to obtain a desired signal-to-noise ratio upon reading. In addition, such materials are oftentimes environmentally unstable, and therefore require the use of additional protective layers thereover. Furthermore, such materials have also proved difficult to make in a controlled fashion, and hence are relatively expensive.
An example of a prior art optical recording material or substrate is described by Chao "New Media Development at Burroughs: Material and Coating", Optical Data Storage, SPIE Vol. 382, pages 149-156. According to this reference, a tri-layer structure is formed which is composed of a reflective mirror layer over which is disposed a quarter wave length dielectric spacer layer over which is disposed a very thin metal absorber layer over which is disposed a thick overcoat protective layer. A preferred example for the metal absorber layer is gold. The gold is deposited in such a way so as to control its thin film microstructure such that the microstructure can be altered by laser heating to form an aggregated discontinuous structure whose reflectivity changes subsequent to being heated. Jipson, "The Writing Mechanism for Discontinuous Metal Films", Optical Storage Media, SPIE Vol. 420, pages 344-48, describes a mechanism by which a reflectivity of a metal absorber layer disposed on a PMMA substrate is changed by laser heating.
Substrates and coatings of the type described and analyzed by these two references are disadvantageous since optical, chemical, and mechanical properties of the metal coating layer are difficult to control during manufacture. Specifically, though the thickness of the metal layer can be varied to vary its absorption, reflection, and transmission, these variables cannot be independently controlled for any given metal to obtain optimum writing recording thresholds or optimum signal-to-noise ratios upon reading. In addition, since metals are excellent heat conductors, metal coatings are disadvantageous since relatively high writing powers are required to locally change the reflectance of the metal coating due to the metal's tendency to rapidly disperse heat over a wide area.